This invention relates to a process of recovering oxyacids of chlorine or salts of such acids by the electrolysis of sea water or of salt solutions contaminated by metal ions.
In the recovery of oxyacids of chlorine or of salts of such acids, such as sodium hypochlorite, by the electrolysis of sea water or of salt solutions contaminated by metal ions, difficulties are still encountered which adversely affect the commercial use and the economy of the process. The calcium or magnesium ions contained in such salt solutions react with the hydroxyl ions formed in the cathode space and such reactions result in a formation of calcium hydroxide and magnesium hydroxide and possibly also of carbonates.
Magnesium hydroxide tends to adhere to the cathode or to deposit on the walls of the electrolytic cell and in that case obstructs the flow of the electrolyte and reduces the efficiency. The growing deposits clog mainly the space between the anode and cathode in the electrolytic cell so that the latter cannot be operated continuously for a prolonged time.
In accordance with known proposals to avoid these difficulties, smooth, uninterrupted metal plates have been used as cathodes in the electrolytic production of hypochlorite and the velocity of flow has been increased and maintained at a certain relationship to the concentration of the electrolyte (German Offenlegungsschrift No. 2,619,497). Known electrolytic cells for the recovery of hypochlorite from sea water have been designed to provide for a directed electrolyte flow at a certain velocity (German Pat. No. 1,956,156).
In the production of alkali chlorates by the electrolysis of alkali chloride solutions, it is known to add alkali hydroxide or alkali carbonate to the electrolyte in order to decrease the current soluble hydroxides, such as calcium or magnesium hydroxide, to the electrolyte and to keep said hydroxide in suspension in the electrolyte throughout the electrolysis (German Pat. No. 90,060).
Finally, it is known to remove undesired metal ions from salt solutions intended for alkali chloride electrolysis and for this purpose to flocculate the ions of iron, magnesium and calcium in the form of carbonates and hydroxides (Swiss Pat. No. 505,751).
It is an object of the invention to decrease or control the formation of deposits on the cathode during the recovery of solutions of oxyacids of chlorine or salts of such acids by the electrolysis of sea water or of salt solutions contaminated by metal ions. In such process, the advantages of known processes should be utilized but their disadvantages should be avoided.